1. Field of the Invention
The present invention relates to heat sinks having cooling fins with end portions which are bonded into slots provided in a base. The present invention also relates to a process for manufacturing heat sinks of this type.
2. Description of the Related Art
Heat sinks having bonded cooling fins generally have rectangular slots extruded into a base, and rectangular fin stock prepared for bonding by abrasive blasting or etching. To aid assembly there is a slight lead-in chamfer at the top of each slot. Thermal conductivity between the fin and the base is maximized by using an adhesive filled with conductive particles, e.g. epoxy filled with aluminum. Since the bond is achieved using an adhesive filled with conductive particles, a minimum gap is needed between the fin and slot wall to prevent the adhesive from being wiped off during insertion of the fin into the slot. This gap is preferably between 0.002 and 0.005 inches or more.
FIG. 1 shows a heat sink according to the prior art, including a base 10 having a rectangular slot 12 with parallel sidewalls 13, and a cooling fin 15 having a rectangular end portion with parallel sidewalls 17. The gap between the cooling fin and the slot is filled with adhesive (not shown).
U.S. Pat. No. 5,554,240 discloses a cooling fin having a tapered plug which is received in a tapered slot machined in a base. The upper thickness of the plug is less than the upper width of the slot, and the plug is tapered with a greater angle than the slot, so that the plug is received in the slot with clearance. This clearance is occupied by particle filled adhesive which is cured while the plug is held in the slot under high pressure.
The present invention is directed to a heat sink including a base having a surface with at least one slot therein, the slot having a pair of mutually facing sidewalls which converge toward each other from the surface to form an angle, and a cooling fin having an end portion received in each slot. The end portion has an end and a pair of mutually opposed sidewalls which converge toward each other toward the end to form the same angle, whereby the mutually opposed sidewalls of the cooling fin are at least substantially parallel to respective mutually facing sidewalls of the slot. The angle is preferably in the range of 5 to 25 degrees.
The key dimensions to control are the convergence angles of the fin and slot, the straightness of the fin, the linearity of the slot, and within-part dimensional uniformity of the slots and fins. Slot depth, slot width, and fin thickness are of lesser importance since they result in fin height variation, which generally has a liberal tolerance. In a typical application, the slot depth might be 0.100 inches. If the convergence angle is 20 degrees, a slot width variation of 0.003 inches will change the fin height by 0.010 inches, which is less than half the traditional design tolerance.
According to a preferred embodiment, a particle free adhesive is used to bond the end portion of the fin to the slot. Use of an unfilled adhesive such as an anaerobic acrylic adhesive not only provides a thinner bond thickness and improved thermal conductivity, but also reduces the cure time and dwell time of the insertion force used to insert the end portion of the fin into the slot. If sufficient force is used to insert the end portion of the cooling fin into the slot, the bond gap can be reduced to the scale of the interfering surface asperities or less. If the surface finish of each part is 30 microinches RA (roughness absolute), the resulting gap could be less than 60 microinches total. This gives a bond that is 50 to 100 times thinner than the current design. Since the thermal resistance of the bond is proportional to its thickness, this reduced gap results in improved thermal performance. With appropriate slot geometry and insertion force, it is also possible to eliminate the use of adhesive altogether.
The slots are preferably machined into the base at a predetermined pitch, i.e., spacing between adjacent slots. This spacing may be uniform across the width of the base, but need not be. Use of horizontal CNC (computer numerical control) mills or high RPM vertical CNC milling machines to create bases from standard shape bar stock permits slot width tolerance control roughly 10 times greater than the slot width tolerance of extruded slots, and 100 times the tolerance control in slot-to-slot spacing on wider shapes. Not only do these improved tolerances make the process more conducive to automation, they allow a closely controlled adhesive bond line thickness. The combination with a precisely machined fin edge provides equivalent or better performance than prior art bonded fins. Manufacturing advantages include self-fixturing, fast cure time, reduced product lead-time (as compared to custom extrusions), and lower tooling costs. These can offset the cost of machining.
Fixtures for holding fins to be inserted in extruded bases are typically made by slicing a piece of the extrusion; this assures that the tolerances will be the same. Since a machining process sets the slot-to-slot pitch with greater accuracy, standard fixtures can be used instead of fixtures made from the extrusion itself. This permits fixture production in parallel (as opposed to in series) with the base production.
The invention provides an essentially flush fit between the end portions of the cooling fins and the sidewalls of the slots. This is accomplished by configuring the mating features such that one final dimension of the assembly is allowed to xe2x80x9cfloatxe2x80x9d so that the tapered fin to base joint can naturally xe2x80x9cfindxe2x80x9d its minimum gap.
Both the slots and the end portions of the cooling fins can be made with substantially the same angles of convergence, or the slot can be formed with a slightly smaller angle of convergence so that it deforms to accommodate the cooling fin and assumes the same angle. This deformation can be elastic, where the sidewalls of the slot deform elastically to conform to the end portion of the cooling fin, or plastic, where the material of the base locally yields to conform to the end portion of the cooling fin. Deformation of the base or the fin reduces the sensitivity to variations in the convergence angles of the fin and the slot.
The slots in the base can be provided with mutually facing sidewalls which both converge toward a central axis which is perpendicular to the surface of the base. Likewise the mutually opposed sidewalls of the end portion of the cooling fin can both converge toward a central axis of the cooling fin, each side converging at a preferred angle in the range of 2.5 to 12.5 degrees. However there are certain inherent advantages in having only one sidewall of the cooling fin converge toward the central axis of the fin, a so-called xe2x80x9csingle side taperxe2x80x9d. These advantages include the possibility of coining the end portion of the cooling fin in a press tool, instead of machining both sides. Where the slot is also provided with a single side taper, the non-converging sidewall of the slot provides a datum face, and the cooling fin is self guiding during insertion. Insertion is also easier due to improved adhesive flow, avoiding hydraulic lock as excessive adhesive is squeezed out. For a single side taper, the preferred convergence is in the range of 5 to 15 degrees.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.